1. Item of Invention
The present invention relates to an apparatus and method for reducing harmonic error power, and more particularly, relates to an apparatus and method for reducing harmonic error power in a self-calibration digital-to-analog converter (DAC).
2. Description of Related Art
Current mode digital-to-analog converters (DAC) are popular in high-speed application. Currents are switched to outputs or ground by switches. The output currents are transformed into voltages by resistors and amplifiers. Open/short operations of switches are controlled by digital signals. However, glitch and harmonic power occur in state transition of switches. There are several approaches for reducing glitches. For example, one approach is to limit bandwidth, by adding a capacitor in a resistor across the amplifier coupled to outputs. Another approach is to “sample and hold” output signals. Or, in still another approach, the output signals are decoded by binary decoding or thermometer decoding. Thermometer decoding is more popular.
Thermometer decoding is not minimal. On the contrary, an N-bit input matches up to 2N outputs in binary decoding. Thermometer decoding has other advantages, such as lower differential nonlinearity (DNL) errors, monotonicity and reduced glitch errors.
Dynamic calibration by current switches is useful in making stable current sources with up to 16-bit precision in audio-frequency DAC. In circuit design, current sources are mismatched because of mismatch transistors and charge injection. For match between current sources, all current sources are periodically calibrated by a reference current source via a shift register. For example, if a current source outputting current Id1 is calibrated, the reference current source calibrates another current source outputting current Id2 as Id1, and so on.
However, in self-calibration, all unit current sources are calibrated to a reference value. After calibration, the currents from the calibrated unit current sources are attenuated linearly. If an offset between one output current and the reference value reaches a half of LSB (least significant bit) or more, that unit current source has to be calibrated again. However, large harmonic power exists in outputs. The harmonic power is related to the calibration periods. That's because the unit current sources are selected by simple and conventional thermometer decoding.